The present invention relates to a circuit for converting a staircase waveform into a smoothed analog signal, and more particularly to a circuit in the form of an improved sample-hold type FM stereo demodulator.
For the stereo demodulator or MPX of FM receivers, there are two types, one being matrix type and the other switching type. The sample-hold stereo demodulator is an improved one of the switching type and has advantages of high channel separation ratio between left and right channel signals and superior dynamic subcarrier suppression. A broadcasting stereo composite signal consists of the addition (L+R) of left and right channel signals, a subcarrier amplitude-modulated by the difference (L-R) of left and right channel signals in carrier suppressed double sideband formula and a pilot signal. In the sample-hold stereo demodulator, two sampling pulse sequences with the same repetition rate as the subcarrier frequency are used. One sampling pulse sequence is in phase with the subcarrier and takes samples of the composite signal for a left channel signal and then holds the taken samples for the sampling interval. The other sampling sequence is in reverse phase with the subcarrier and takes samples for a right channel signal in a similar manner. The output signals of the sample-hold circuit are staircase waveforms of the left and right channel signals. Although the staircase waveform contains less undesired components as compared with a series of samples obtained by an ordinary switching means, for high-fidelity reproduction a sophisticated lowpass filter is still required to completely eliminate the residual subcarrier component and other spurious frequency components from the staircase channel signal without introducing any distortion for amplitude and phase characteristics of the channel signal. It is considered that, if the staircase waveform is further approximated to the original analog waveform shape of a channel signal, such severe requirements for the filter will be relieved. The Japanese Utility Model Application No. 54-157501 filed Nov. 13, 1979 by the same assignee disclosed a simulated analog signal converting circuit for digital signal like a staircase waveform in which each stepping height of the staircase is linked with a flight of more finely stepped stairs.
The object of the present invention is to provide a more improved circuit for converting staircase waveforms into smoothed analog waveforms.